Observational evidence for intermediate-mass black holes

Abstract

Intermediate-mass black holes (IMBHs), with masses in the range 100 − 10 6 M , are the link between stellar-mass BHs and supermassive BHs (SMBHs). They are thought to be the seeds from which SMBHs grow, which would explain the existence of quasars with BH masses of up to 10 10 M when the Universe was only 0.8 Gyr old. The detection and study of IMBHs has thus strong implications for understanding how SMBHs form and grow, which is ultimately linked to galaxy formation and growth, as well as for studies of the universality of BH accretion or the epoch of reionisation. Proving the existence of seed BHs in the early Universe is not yet feasible with the current instrumentation; however, those seeds that did not grow into SMBHs can be found as IMBHs in the nearby Universe. In this review I summarize the different scenarios proposed for the formation of IMBHs and gather all the observational evidence for the few hundreds of nearby IMBH candidates found in dwarf galaxies, globular clusters, and ultraluminous X-ray sources, as well as the possible discovery of a few seed BHs at high redshift. I discuss some of their properties, such as X-ray weakness and location in the BH mass scaling relations, and the possibility to discover IMBHs through high velocity clouds, tidal disruption events, gravitational waves, or accretion disks in active galactic nuclei. I finalize with the prospects for the detection of IMBHs with up-coming observatories.

On monolithic supermassive stars

“…Whether merger events are able to lead to eruptive episodes that partially or fully unbind the star requires considerable further investigation. In the event of an absence of additional "islands of explodability," such as the narrow range of masses found for helium-burning supermassive stars by Chen et al (2014), evidence for the mass distri-bution of supermassive stars, and their final fates, may need to be inferred from the results of future efforts to find intermediate mass black holes (Mezcua 2017;Koliopanos 2017;Woods et al 2019), massive black hole seeds accreting at high redshift (e.g., Pacucci et al 2019), and critically, direct detections of supermassive stars themselves (Surace et al 2018(Surace et al , 2019.…”

Black hole feedback and the evolution of massive early-type galaxies

“…Sijacki et al 2007;Booth & Schaye 2009), more energy would have been injected via AGN activity in those objects hosting more massive black holes. In practice this means that the distance of a galaxy from the average M•-σ relation (∆M•) directly correlates with the (relative) amount of black hole feedback experienced during its evolution, independently from the initial mass of the black hole seeds (Mezcua 2017). Therefore, by comparing the properties of galaxies above and below the M•-σ relation, so-called over-massive and under-massive black hole galaxies, respectively, one can empirically assess the effect of black hole feedback on the baryonic cycle of galaxies.…”

Introducing the Search for Intermediate-mass Black Holes in Nearby Galaxies (SIBLING) Survey

“…More recently, gravitational waves produced by BH binary mergers have been detected by LIGO (e.g. Abbott et al 2016, 2017 with M BH < 50 M per-merger and up to 80M for the remnant. On the other side of the mass spectrum, the existence of supermassive black holes (SMBH, M BH 10 6 M ) in the center of galaxies has been proven by many studies (e.g.…”

“…Observations of luminous quasars at redshift z ∼ 7 hosting SMBH with masses up to ∼ 10 10 M (Willott et al 2007;Mortlock et al 2011) impose some restrictions to the formation of this SMBH and their seeds, however, they represent exceptional systems and very possibly not the norm. In the SMBH formation scenario there are mainly three possible paths (Volonteri 2010;Reines & Comastri 2016;Mezcua 2017): (i) BH seeds formed from the death of first generation stars producing BHs with masses of ∼ 10 2 M ; (ii) via runaway collisions/mergers of stars in dense stellar cluster producing BH seeds of 10 2 −10 4 M ; or (iii) from the direct collapse of large and dense cold gas in the early Universe forming BHs with masses between 10 5 − 10 6 M . These processes create a population of IMBHs with masses between 10 2 − 10 6 M , which are still largely undetected.…”